Do night shifts cause breast cancer?

So far only one country (Denmark) has awarded compensation to shift workers who develop breast cancer, but if more followed suit, it could have far-reaching economic consequences.

Whether breast cancer should be labeled as an occupational disease is still unclear, researchers behind a new study from China suggest.

Breast cancer is the most common cancer in women worldwide, and so far only one country (Denmark) has awarded compensation to shift workers who develop the disease. If more were to follow suit, it could have far-reaching economic consequences.

The new report, however, showed no sign of a connection between night-shift work and breast cancer.

"We basically found no association, even among women who had more than 25 years of shift work," said Wong-Ho Chow, a researcher with the National Cancer Institute in Bethesda, Maryland, who worked on the study, published in the American Journal of Epidemiology.

With colleagues from US and Chinese universities, Chow followed more than 70,000 women in Shanghai, inquiring about their work and cancer status.

First study from a non-Western country

Earlier research in animals had suggested that disturbances in the light-dark cycle could knock certain chemicals in the body -- for instance, melatonin and oestrogen -- off balance, and as a result increase cancer risk.

"We wanted to explore this hypothesis," Chow said.

Over a decade, about one in a hundred women in the study developed breast cancer. But the risk didn't increase for women who worked nights, regardless of how long they had done so. There is really not much that you can criticise from reading the paper," said Dr. Kurt Straif, an expert in occupational medicine, who was not involved in the research.

But, he added, "One has to keep in mind that this is the first study from a non-Western country."

In 2007, a team headed by Straif at the World Health Organization's International Agency for Research on Cancer had concluded that night-shift work was a probable cause of cancer.

What the study showed

Based on animal research as well as studies of nurses and flight attendants, the team listed shift work in the second-highest cancer-risk category, group 2A, which includes exposure to tanning beds, crude-oil refinement and hairdressing agents.

Although the evidence wasn't clear cut, Straif said, six out of eight studies found an increased risk of at least 50%.

Denmark reacted to the team's conclusion by granting breast cancer the status of occupational disease. In 2009, the Scandinavian country began awarding compensation to affected women who had no other obvious risk factors.

"We have a pretty expansive attitude toward recognizing occupational hazards," said Johnni Hansen of the Danish Cancer Society, who has spent a decade studying shift work and breast cancer.

"In most other countries," he said, "you need to have a more solid connection."

He added that the compensation was still fairly limited, averaging only about $10,000 to $12,000 per case.

Both Straif and Hansen said that at this point, it is too early to consider changing the World Health Organisation's classification of night-shift work.

For one thing, the specific type of shift work -- rotational or regular -- could make a difference. In addition, the study followed only people who were healthy to begin with, and so may have missed some long-term effects that would only show up after the follow-up ended. And finally, environmental and genetic differences between West and East could in principle play a role.

Straif said several new studies were underway. While he declined to comment on Denmark's policy regarding breast cancer cases, he said he "would be more worried about the many countries that do not compensate these people." - (Reuters Health, May 2010)

Ask the Expert

Dr Gudgeon qualified in Birmingham, England, in 1968. She has more than 40 years experience in oncology, and in 1994 she founded her practice, Cape Breast Care, where she treats benign and malignant breast cancers.
Dr Boeddinghaus obtained her qualification at UCT Medical School in 1994 and her MRCP in London in 1998. She has worked extensively in the field of oncology and has a special interest in the hormonal management of breast cancer. She now works with Dr Gudgeon at Cape Breast Care. Read more.

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